Fiber production process

ABSTRACT

A method of forming a flame retardant cellulose fiber is disclosed which comprises the steps of producing lyocell fiber and incorporating a flame retardant chemical into the fiber while the fiber is in the never-dried condition prior to first drying.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods of producing fibre and has particularreference to methods of producing fibre having inherent flame retardancyproperties.

2. Description of the Related Art

As used herein, the term "lyocell" is defined in accordance with thedefinition agreed by the Bureau International pour la Standardisation dela Rayonne et de Fibres Synthetique (BISFA) namely:

"A cellulose fibre obtained by an organic solvent spinning process; itbeing understood that:

(1) an "organic solvent" means essentially a mixture of organicchemicals and water; and

(2) "solvent spinning" means dissolving and spinning without theformation of a derivative".

As used herein, by a "flame retardancy chemical" is meant one whichretards the burning of a product to which it is applied.

SUMMARY OF THE INVENTION

The present invention provides a method of producing a flame retardantlyocell fibre which comprises the steps of:

(i) forming a solution of cellulose in an organic solvent,

(ii) extruding the solution through a spinnerette downwardly into an airgap to form a plurality of strands,

(iii) passing the thusly formed strands downwardly through awater-containing spin bath,

(iv) leaching the solvent from the thusly formed strands to producefilaments of cellulose,

(v) incorporating into the filaments of cellulose, whilst still wet, aflame retardant chemical, and

(vi) fixing the chemical onto the cellulose to produce a cellulosefilamentary material having inherent flame retardancy.

The present invention further provides a method of forming a flameretardant cellulose fibre comprising the steps of producing lyocellfibre and incorporating a flame retardant chemical into the fibre whilstthe fibre is in the never-dried condition (i.e. prior to first drying).

The flame retardant chemical may be a phosphorous based chemical and maybe a quaternary phosphonium compound. The flame retardant chemical maybe tetrakis (hydroxymethyl) phosphonium salt.

The flame retardant chemical may be fixed by a curing process utilisingthe action of ammonia or heat. The flame retardant chemical ispreferably applied to never-dried lyocell fibre in tow form. The tow maybe cut into staple fibre prior to drying for the first time or afterdrying.

The tow having the flame retardant chemical or chemicals fixed thereonmay be dried as tow, crimped and cut to form staple fibre. The tow maybe provided with a finish, a chemical compound added to the tow toenhance or ease the processing of fibre during subsequent operations.The fixing of the flame retardant chemical to the cellulose may becarried out during the drying of the cellulose, or may be carried out asa separate step prior to the drying of the cellulose. Alternatively, thecellulose may be dried and then passed through a fixing process finallyto fix the flame retardant chemical to the cellulose.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example the present invention will now be described withreference to the accompanying drawings.

FIG. 1 shows schematically an application route for the application offlame retardant (FR) PROBAN precondensate chemicals to fibre.

FIG. 2 shows schematically an application route for the application ofFR PYROVATEX chemicals to fibre.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The production of lyocell fibre is described in U.S. Pat. No. 4,416,698,the contents of which are incorporated herein by way of reference.Lyocell fibre may be produced by any known manner. The invention issolely concerned with the production of a flame retardant lyocell fibre.

DESCRIPTION OF PREFERRED EMBODIMENTS

In a preferred process for the production of lyocell fibre, a solutionof cellulose in an organic solvent, typically N-methyl morpholineN-oxide is formed by heating N-methyl morpholine N-oxide, water andcellulose to evaporate the water so as to form the solution. Thesolution may contain a suitable stabiliser. The solution is commonlyreferred to as a spinning dope. This dope is then forced through aspinnerette jet to pass in filamentary form as strands through an airgap into a spin bath. The spin bath contains water and leaches thesolvent from the strands. During the leaching process the cellulosecomponent of the solution re-forms to produce the cellulosic filamentarymaterial. The filamentary material is in the form of a bundle offilaments, commonly referred to as a tow. The tow comprises essentiallya plurality of parallel filaments, the number of filaments in the towbeing equal to the number of strands produced by the spinnerette jet.

The tow of fibre having been produced by the leaching process isreferred to as never-dried fibre, in the sense that the tow is still wetand has not been dried at that stage in its processing life. Never-driedfibre has slightly different physical characteristics to fibre which hasbeen dried and is subsequently rewetted. Typically never-dried fibrecontains a greater proportion of water than can be incorporated intodried fibre merely by wetting it.

One type of flame retardant treatment is the PROBAN precondensatetreatment using tetrakis (hydroxymethyl) phosphonium (THP) availablefrom Albright & Wilson Ltd., England.

The never-dried fibre is then treated to give it a PROBAN precondensatefinish in accordance with the sequence illustrated in FIG. 1. The fibreis first passed through a bath containing PROBAN pre-condensate namely amixture of tetrakis (hydroxymethyl) phosphonium and urea. The fibreemerging from the bath is then passed through the nip of a pair ofrollers to remove excess pre-condensate. This is the process illustratedby block 1 in FIG. 1. The fibre is then passed through an ammoniasolution or has ammonia sprayed onto it in box 2A. The thus treatedfibre is then dried at 130° C. in a suitable drying equipment such as adrying tunnel or by being passed over heated drying rollers. The drying,at a temperature of 130° C. occurs in block 2B. In an alternative formof curing process, blocks 2A and 2B are replaced in their entirety by aheat cure step which occurs at 120°-170° C.

After the precondensate has been applied and cured onto the fibre it isoxidised as at block 3 using, for example, hydrogen peroxide solution.

The oxidised coating is then neutralised as at block 4 with, forexample, a solution of sodium carbonate.

Subsequently the fibre is washed as at block 5 and is then passedthrough a soft finish roller as at block 6 prior to drying as at block7.

The solutions of hydrogen peroxide, sodium carbonate or similar and softfinish can be applied either by dipping the fibre through the solutionor by spraying a solution onto the fibre or by an other suitable means.Typically the fibre is washed by plating the fibre onto a porous supportsuch as a steel mesh and then washing with demineralised water. Thefibre is dried by suitable dryers such as drum dryers.

In an alternative process, PYROVATEX solution may be applied to thenever-dried fibre. This process is illustrated in block form in FIG. 2.In this case the PYROVATEX solution is applied to the fibre at 8 bydipping the fibre in PYROVATEX solution, a fixing resin such as LYOFIXResin and phosphoric acid. Subsequently the excess solution on the fibreis removed by passing the fibre through the nip of a pair of rolls. Thefibre is then dried at 130° C. at 9 and cured in a separate curing ovenat 160° C. for 5 minutes as shown at block 10. Subsequently the fibre istreated with sodium carbonate solution to neutralise the fibre as atblock 11, washed as at block 12, has a soft finish applied to it as atblock 13 and is then dried as at block 14. The solutions and dryingprocesses described in connection with FIG. 2 would effectively be thesame as those used in connection with the processed illustrated inconnection with FIG. 1.

Once the never-dried fibre has been treated with THP or other treatmentand cured it can then be dried in a conventional manner. The fibre ispreferably washed prior to drying to remove excess THP from the fibre.The fibre can be dried either in tow form and utilised as tow, or it canbe dried in tow form and subsequently cut to staple. Optionally thefibre may be crimped after drying by means of a mechanical crimpingprocess, and then cut to form staple.

Alternatively, the fibre after curing may be cut to form staple, washedand dried as staple.

The flame retardant chemical may be applied to the fibre in staple formrather than in tow form. Thus after the leaching operation the fibre canbe cut to form staple, washed, and the flame retardant chemical can thenbe applied to the staple. The staple can then be cured, washed and driedas staple. It is preferred, however, that the FR chemical be applied tothe fibre in tow form because it is found that there is less entanglingof the fibre and the tow treated fibre may be more readily carded toproduce an open structure suitable for spinning. The treated fibre canthen be processed in a conventional manner to produce fabric. In thecase of filamentary material the filament would be wound up andconverted by weaving or knitting or non-woven methods to produce afabric. In the case of staple fibre, the fibre would be carded, spun andthe yarn produced by spinning could be woven or knitted to produce asuitable fabric. The fabric may be dyed either after production or itmay be dyed as yarn to produce a coloured yarn for the production offabric.

Rather than using THP or other phosphorous-based compounds--typicallyquaternary phosphorous--based compounds, nitrogen-based compounds can beused or any other suitable flame retardant.

By incorporating the flame retardant chemical into the fibre in thenever-dried state, it is possible to produce fibre which is inherentlyflame retardant when tested in accordance with British Standard 5867 andwhich produces fabrics having very good flame retardancy properties. Thefibre can be treated on-line under controlled conditions and thecustomer need not carry out any subsequent flame retardancy treatment tohave a flame retardant fabric. It is believed that never-dried fibrepicks up about 75% by weight of the active phosphorous containingingredient compared to a pick-up of about 30% by weight for dried fibre.

In a test, two samples of lyocell fibre were produced, one was dried andtreated with 50% (by weight) PROBAN pre-condensate followed immediatelyby padding with a soft finish, CROSOFT XME finish at 20 g/l. The treatedfibre was then dried at 70° C., cured in ammonia gas at ambienttemperature, oxidised with hydrogen peroxide solution, neutralised withsodium carbonate, washed and dried. The other sample was given the sametreatment, but the treatment was applied to lyocell fibre which hadnever been dried before the PROBAN precondensate and CROSOFT XME finishwere applied.

The following results were obtained as set out in Table 1:

                  TABLE 1                                                         ______________________________________                                                           Never Dried                                                                           Dried                                              ______________________________________                                        1.     Tensiles                                                                      Tenacity (cN/tex) 34.05     30.64                                             Extension (%)     9.070     7.56                                              Dtex              2.129     2.20                                       2.     Flame Retardancy                                                              % LOI             31        28                                                % Phosphorus (V)  4.15      2.46                                              % Phosphorus (III)                                                                              1.0       0.5                                               % Nitrogen        3.99      2.27                                              Formaldehyde (ppm)                                                                              170       180                                        3.     Additive Pick Up/Distribution                                                 Dry pick up (g/g) 0.45      0.28                                       ______________________________________                                    

It can be seen, therefore, that the application of the PROBANprecondensate treatment to the never dried fibre not only significantlyincreases the LOI compared to the application to dried fibre, but thatthis is also accompanied by better tensile properties.

It can be seen that the phosphorus pick up in the never dried fibre ishigher than in the dried fibre, and this is confirmed by elemental mapmicrographs. Comparing the elemental phosphorous maps across theindividual fibres by means of line scans shows that there is aconcentration of phosphorus in the skin of the dried fibre treated withProban, whereas the fibre treated in the never dried condition shows amuch more even distribution across the fibre.

We claim:
 1. A method of forming a flame retardant cellulose fibrecomprising the steps of producing lyocell fibre and incorporating aflame retardant chemical into the fibre whilst the fibre is in thenever-dried condition prior to first drying.
 2. A method as claimed inclaim 1 in which said method includes the steps of:(i) forming asolution of cellulose in an organic solvent, (ii) extruding the solutionthrough a spinnerette downwardly into an air gap to form a plurality ofstrands, (iii) passing the thusly formed strands downwardly through awater-containing spin bath, (iv) leaching the solvent from the thuslyformed strands to produce filaments of cellulose, (v) incorporating intothe filaments of cellulose, whilst still wet, a flame retardantchemical, and (vi) fixing the chemical onto the cellulose to produce acellulose filamentary material having inherent flame retardancy.
 3. Amethod as claimed in claim 1, in which the flame retardant chemical is aphosphorus based compound.
 4. A method as claimed in claim 3, in whichthe flame retardant chemical is a quaternary phosphonium compound.
 5. Amethod as claimed in claim 4, in which the flame retardant chemical is atetrakis (hydroxymethyl) phosphonium salt.
 6. A method as claimed inclaim 4 in which the flame retardant chemical is fixed by a curingprocess utilising the action of ammonia or heat.
 7. A method as claimedin claim 1, in which the flame retardant chemical is applied to thefibre in tow form.
 8. A method as claimed in claim 7, in which the towis cut into staple fibre prior to drying for the first time, or afterdrying.
 9. A method as claimed in claim 1, in which the flame retardantchemical is fixed to the cellulose prior to, during, or after drying.10. Cellulose fibre produced by the method of claim 1.